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Proposes a model for kinetic processes said to be similar in computational effort and yielding similar results to conventional transition state theory (TST), while maintaining parsimony and credulity. Argues that partitioning of states into groups be limited to energy space in contrast to TST. (JM)

The kinetic-molecular model (KKM) for calculating the rates of condensation of supersaturated unary vapors has been extended to ideal and nonideal binary vapors by treating such mixtures as pseudovapors with properties assigned according to a linear combination of the properties for the pure components in the vapor phase. The single empirical KMM parameter, the hot-cluster stabilization parameter (SP) was found to vary with vapor composition, with deviations from linearity that could be correlated with cluster compactness; indeed, SP provides a measure of this structural feature. [ABSTRACT FROM AUTHOR]

When B5H9 is injected into a stream of He that is carrying O(3P) atoms (approximately 100/1), at a total pressure of 5–15 Torr, a blue-green flame develops. The major chemiluminescent species is BO(A 2Π). While its translational and rotational temperatures are ≊350 K, the vibrational temperature in the A state is high, ≊3800 K. From among the many products of this reaction, the OH radical can be most easily quantitated by measuring the intensity of its laser-induced fluorescence. The central streamline from a flow-tube reactor was extracted into an evacuated plenum via a pinhole. The time-intensity profile was calibrated using C2H6 for the fuel. Check runs were made with B2H6. A multistep mechanism was developed for B5H9+O(3P) that simulates the shape as well as the magnitude of the OH concentration over a reactor residence time 0.5–10 ms. Less than a dozen crucial reactions were identified by means of an extended sensitivity analysis. Breakdown schemes for the oxidation of B2H6 and B5H9 have been developed. [ABSTRACT FROM AUTHOR]

The rate of bond inversion at the nitrogen atom in aziridine (dimethylene-imine) was measured in the gas phase for a range of pressures and temperatures. 1H NMR spectra were recorded of samples at 5–1100 Torr, and from 298 to 388 K. Rate constants for inversion were derived by complete line shape analysis of the NMR spectra. Their temperature dependence at the high pressure limit (1100 Torr) gave Ea =15.8±0.4 kcal mol-1; ΔH‡ =15.2±0.4 kcal mol-1; ΔS‡=-5.5±3 eu; and ΔG‡298=16.8±0.5 kcal mol-1. These values are in excellent agreement with previously reported mangitudes obtained for solutions and in the gas phase at high pressures. However, at all temperatures we found no dependence of the first order rate constants on pressure down to 5 Torr, in sharp contrast to the fall-off curves calculated with standard RRKM codes, which indicated that for pressures below 100 Torr this system should be in the bimolecular regime. A less extended set of measurements was made with 2-methyl-aziridine which does show a small but clear fall off in kuni. These results can be rationalized in terms of the regional phase space model, previously proposed for methylnitrite. In systems such as aziridine, where the density of states at the barrier height is sparse (approx. 10/cm-1) the rate of inversion is limited by the relaxation time for intramolecular vibrational energy redistribution. Our results indicate that many nanoseconds are required to attain relaxation rather than picoseconds, estimated from the Marcus formula for k23(E)=N‡(E+)/hρ(E). In the methyl-substituted analog the barrier height is approximately 3 kcal mol-1 higher, and the density of states at this barrier is higher than in aziridine. Relaxation data for conformational changes in C6H12, H3CONO, SF4, and C5H10O were reanalyzed in terms of the regional phase-space model. At low pressures these also show departures from RRKM predictions. [ABSTRACT FROM AUTHOR]

The temperature and density in an overexpanded jet (Ar) were mapped by the method of characteristics. The kinetics of condensation in the supersaturated gas along the central streamline was analyzed on the basis of four models, subject to previously developed thermodynamic relations which must apply during the homogeneous condensation of any supersaturated vapor. Here we report on detailed dynamic analyses of the rates of cluster growth. Each association: An-1+A1⇋An was represented either by a single stabilization step, or by a sequence of two energy removing collisions, for which kinetic parameters based on Lennard-Jones (LJ) potentials were estimated. Coupled differential rate equations (up to 192) were solved, both for cases when the heat of condensation was neglected, and when it was partially or fully distributed throughout the fluid. The distribution of cluster sizes declines exponentially, according to N(n)=A exp(-an)+B exp(-bn), in agreement with generally observed distributions for monatomic species. For source pressures of 3 and 10 atm the condensation process becomes kinetically controlled within a distance downstream somewhat less than twice the aperture diameter. Our calculations indicated that after about three aperture lengths ‘‘freezing in’’ takes place if the heat of condensation is neglected. When the heat of condensation is distributed (partitioned in one of several plausible ways) freezing in is delayed beyond four aperture diameters. A modest fraction of the nascent clusters retain some of the heat of condensation and remain at a higher temperature than the ambient gas. [ABSTRACT FROM AUTHOR]

The dynamics of the trans⊌cis isomerization of monothioformic acid (rotation about the HC–SH bond) is a case study for an intramolecular conversion over a significant barrier [microwave spectra indicates Vmax(θ) =3500 cm-1], where the density of vibrational states at the potential maximum is very low, approx. 2.5/cm-1. We recorded the NMR spectra of dilute solutions of the acid in CD2Cl2, and of the gas at low pressures, over the temperature range 300–170 K, to determine the exchange relaxation time and to deduce the corresponding kinetic parameters. In spite of difficulties with the preparation and maintainence of the pure material, (i) correct assignment of the NMR resonances were made; (ii) the magnitude of E0sol for isomerization in solution, from k∞u, was found to be compatible with the barrier derived from microwave spectra; (iii) in the gas phase this intramolecular conversion is in the second order regime, but (iv) only an upper limit could be set on the magnitude of E0gas for isomerization, which is at least 2 kcal/mol lower than E0sol. [ABSTRACT FROM AUTHOR]

We describe a modified configuration of the temperature drift (TD) technique for recording infrared spectra that arise from vibrationally excited states. TD is a differential measurement that records decrements in IR absorptions due to modest increases in temperature. In particular, it permits identification of oscillators that are coupled to large amplitude, low frequency motions associated with transition structures, such as transitions to free rotations about single bonds that are hindered at ambient temperatures. Selected portions of the spectra of two species were recorded to demonstrate the feasibility of locating spectral features of critical transition structures. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We measured, via LIF, time-dependent concentrations of OH resulting from the reaction of O(3 P) with B2H6 and C2H6. The oxygen atoms were generated by titrating microwave discharged N2 /He with NO to the chemiluminescent end point. The operating pressures in the flow reactor ranged from 5 to 15 Torr; the mixtures consisted of He/O(3 P)/fuel in the approximate ratios 100/1/0.1 to 100/1/1. Flow conditions were such that in the low pressure experiments the controlled residence time prior to detection were 0.8–17 ms; under the higher pressure conditions, the time interval covered was 2–35 ms. We estimated that the temperature of the reaction region was approx. 350 K, based on rotational emission temperatures measured for BO*, generated under closely similar conditions. First a complete mechanism was derived for ethane for a specified set of experimental parameters. For ethane, a single set of experimental conditions was selected for ratioing the recorded intensity to the computed OH density; this was cross checked with other runs for ethane. Finally, a mechanism was developed for B2H6, which quantitatively checked our experimentally determined profiles both in shape and magnitude, for three sets of conditions, and within a factor of 2 for the high concentration runs (100% B2H6 feeding into the reactor at 14 Torr). [ABSTRACT FROM AUTHOR]

The gas-phase spectrum of BH3 was detected for the first time by diode laser spectroscopy through the observation of the ν2 band. The borane was produced by photolysis of B2H6 or BH3CO with an ArF excimer laser. The same spectrum was obtained by a discharge in a B2H6 and He mixture. Line assignments were made for the Q-branch series with J=K of the ν2 band; effects due to Coriolis perturbations by the ν4 state were recognized. [ABSTRACT FROM AUTHOR]